Almost 600 million years ago, the Earth was largely or entirely locked in deep ice, with temperatures so cold even the equator froze solid, a period known as “Snowball Earth”. There is plenty of debate on what allowed the Earth to escape the snowball, with new evidence crediting a particular type of volcano.
Once the Earth became cold enough, it was hard for it to warm. The sheets of ice covering almost the entire planet reflected so much sunlight that the planet's temperature remained far beneath the point where a small increase in brightness from the Sun, or chances to the Earth's orbit, could break the spell. Yet somehow, something did, in the process triggering a vast expansion of life to fill the new niches.
The most likely explanation appears to be an increase in volcanism. This would be somewhat ironic, since it is also suspected volcanoes put the Earth in the freezer in the first place.
The explanation for this apparent contradiction lies in the face that volcanoes release both cooling gasses, like sulfur dioxide, and warming carbon dioxide. The ratio of these varies greatly between eruptions. According to Dr Chad Deering of Michigan Technological University, the end of Snowball Earth began with very carbon dioxide-rich eruptions.
The evidence, presented in Nature Geoscience, is found in small crystals called zircons from a 3,000-kilometer (1,860-mile) swath of Antarctica. Zircons are useful to geologists because they exclude lead when they form, but include uranium, which then radioactively decays into lead at known rates. Consequently, ratios of lead to uranium reveal a zircon's age.
Examining zircons that formed between 500 and 700 million years ago, Deering found a spike 553-577 million years ago from alkaline volcanoes. Modern equivalents (including Mount Etna and Mount Erebus) release 10-50 times as much carbon dioxide, relative to the size of their eruptions, as other volcanoes. Such a large release would warm the planet enough to trigger a feedback loop, where loss of ice allowed more sunlight to be absorbed, leading to yet more melting.
The magma in eruptions comes from mantle material deep in the Earth forcing its way to the surface. However, tectonic interactions can cause the mantle to become “polluted” with water, gas, and carbon from the Earth's crust. Alkaline volcanoes contain particularly polluted mantle material.
"In essence, we discovered that throughout the Earth's history there was a particularly significant pulse of carbon emitted that immediately preceded the Cambrian explosion, the most important emergence of life that has yet to occur," Deering said in a statement. Alkaline volcanic activity appears to be strongly associated with continental rifting, when the continental crust pulls apart until mantle can get through.
